Specific challenges in the application of fibre-metal laminates
Fibre-metal laminates as a combination of metals and fibre reinforced plastic materials are investigated in a variety of current research projects. The intention of combining these two different materials is the compensation of their inherent weaknesses. However, their application as multi-layered laminate is also accompanied by a wide range of challenges, as DLR and TU Braunschweig scientists describe.
Intrinsic Hybrid Laminate
Fibre reinforced plastic (FRP) composites meet the requirements of many different lightweight applications due to their extraordinary weight-specific stiffness and strength as well as their marginal fatigue. However, they also show some major weaknesses like low bearing strength or brittle failure properties. Fibre-metal laminates (FML), consisting of fibre reinforced plastic and metal, are often investigated with the aim to overcome these weaknesses. Although many different definitions for FML can be found, the major difference to hybrid structures or composite constructions is that the material combination affects the laminate architecture. This means that an FML consists of at least one FRP and one metal layer. Additionally, the term ’intrinsic hybrid laminate’ has been established to describe those FML where the cohesion between FRP and metal is created by the FRP matrix during its cure without the use of any additional adhesive.
Unfortunately, it can be observed that FML are taken into consideration for former metal parts which are made of FRP without changing their geometry. This approach is strikingly called ’black metal’ design as carbon fibre reinforced plastic (CFRP) is used in the geometry of a metallic part instead of adapting the design to the specific demands of FRP with regard to mechanics and manufacturing. Intrinsic hybrid laminates are than employed to compensate the inappropriate design.
The selection of the FML based on the desired properties and a subsequent detailed design is more adequate and recommended. However, it must be considered that the material properties of an FML do not depend on the material combination only. The material fractions and in particular the layer thickness of both constituents are of major influence as well.
The best known FML consists of glass fibre reinforced plastic (GFRP) and aluminium: Glare. The purpose of the GFRP layers is to bridge fatigue cracks induced in the aluminium and to reduce their progress . Therefore, parts under cyclic tensile load, like the upper central fuselage panels of the Airbus A380, are preferred for its application. The glass fibres in Glare are preferably oriented along the main load direction or as a bidirectional laminate.
The selection of the FML based on the desired properties and a subsequent detailed design is more adequate and recommended.
A further application for ’intrinsic hybrids’ are UD-CFRP-steel laminates. Especially, when high specific uniaxial mechanical properties are aspired, notch and impact sensitivity properties limit the fibre fraction in load direction drastically. Therefore, the laminates loose a certain fraction of their lightweight potential mainly due to the residual strength considerations. Recognising these limitations, UD-CFRP-steel laminates aim to reduce the aforementioned disadvantages by adding steel layers with a thickness below 0.08 mm up to a maximum metal volume fraction (MVF) of 12 % steel. These layers do not reduce stiffness and strength. They even increase the residual strength of previously impacted laminates. Tests show an increase of weight-specific stiffness and strength up to 15 % at a comparable residual compression strength after impact level . At the same time, the weight-specific residual compression strength of a laminate with identical damage size is increased significantly.
The creation of an FML can also be limited to a certain area of a part by adding FRP locally to a metallic part or by adding metal locally to an FRP part. The material’s exploitation of FRP in a part, especially when using CFRP, is often limited by its low bearing strength and it may only be exploited to its full extent when adding a ramp-up to the part. By adding metallic inserts, the laminate’s bearing strength increases and a local FML is created. An even higher load carrying capacity is achieved when multiple metallic layers are used . In this case, FRP-layers are substituted by titanium or stainless steel sheets, whereby three different areas are created: a pure FRP area, a hybrid region with the maximum metal content and a transition region in between the first two areas. With the help of a certain metal content and a structural design of the transition region, the load bearing capacity can be increased in a way that no additional ramp-up is required [3, 4, 5].
An intrinsic hybrid is also created by using only one metal layer, for example adding a metallic abrasion protection layer to the outer surface of an FRP part  or integrating metallic conductive tracks in the FRP laminate.
’Intrinsic hybrid laminates’ offer a variety of parameters with regard to their lay-up. Besides the material of the two constituents, their content, single layer thickness and their arrangement are variable. The metal volume fraction (MVF) is widely used to describe the material content.
A simple analytic estimation of stiffness and strength depending on the MVF can be done by using rule of mixtures or classical laminate theory (CLT). The first method does not consider the suppression of the transverse contraction evoked by adjacent layers, and hence, is not recommended for low MVF .
Thereby, the weight-specific stiffness and the compression strength of certain material combinations can be calculated with low effort, as exemplary shown in
Some essential characteristics of the material combinations can be derived by such a simple approach, as Figure 4 for example, indicates that a high laminate compression strength increase is received as a consequence of residual stress for the combination of CFRP and aluminium at low MVF. However, these stresses lead to a significant decrease in tensile strength and prohibit a reasonable use of this material combination.
The design of the interface between matrix and metal surface has a large impact on the material’s behavior. In addition, the metal layers serve as a barrier layer against fluids. Although prepregs are used mostly for the manufacturing of intrinsic hybrid laminates, depending on the required flow distance, resin infusion techniques are applicable as well.
The development of the surface treatment is essentially performed empirically and hence, requires the evaluation of the adhesion. In general, this evaluation is done by determining the interlaminar shear strength with the help of the 3-point-bending test, the 5-point-bending test or the double-lap shear test. An alternative is to use the crack energy release rate for evaluation. The main disadvantage of most of the specimen geometries is that residual thermal stresses superimpose the stresses generated by the test load and falsify the results. The mentioned bending tests show an advantage as the larger residual shear stresses in the specimen`s longitudinal direction do not act in the loaded area . However, when applying DIN EN ISO 14130 to determine the interlaminar shear strength, the special characteristic of the shear stress in FML needs to be taken into account, which is a consequence of the different single layer stiffnesses. In addition, testing at different temperatures and different moisture contents is recommended.
For the combination of steel 1.4310 and thermoset CFRP, the vacuum blasting process achieved significantly higher interlaminar shear strength than the reference and proved to show high potential for automation .
Thermal Residual Stresses
Residual thermal stresses occur as a consequence of the difference in CTE of the two constituents of an ’intrinsic hybrid’ and the difference between cure temperature of the matrix and operational temperature of the cured laminate. In non-symmetrical laminate lay-ups, inhomogeneous residual stresses are generated which result in deformations. As further effects occur as well and may lead to thermal stresses and deformations, they must be differentiated systematically to develop analytical and numerical methods for their evaluation.
By introducing a cooling step the residual thermal stresses can be reduced by approximately 25 %.
A combined approach using FBG sensors and warping evaluation of non-symmetric strips has been developed at the DLR. Non-symmetric specimens are created by removing certain layers after cure, Figure 6. A quantitative evaluation is achieved by determination of their curvature and measuring the FBG strain at room temperature .
By introducing a cooling step in the curing cycle and taking advantage of the exothermal cross-linking reaction`s inertia, the residual thermal stresses in an FML using the above mentioned matrix system can be reduced by approximately 25 %.
The application of nondestructive test methods on a material is a crucial requirement for its use. In aerospace, the inspection is performed visually with or without any auxiliaries or ultrasonic. Computer tomography (CT) is applied for very few more detailed inspections.
The visual detection of impacted parts is simplified by the use of FML as, in contrast to pure CFRP, failures in the laminate are indicated by a detectable failure on the laminate surface. However, due to the large number of layers with different acoustic impedances, ultrasonic inspection by pulse-echo is only suitable to a limited extent. CT however, is also accompanied by severe streaking artifacts that occur due to the fact that the high density of the metal results in incomplete attenuation profiles. This overranging can be reduced by means of special software corrections, but a loss of detail around the metal interface remains, even though different physical filters have been tested. As a result, the effort to characterise the failure’s geometry is increased and impedes the better understanding of the involved failure mechanisms.
Different advantages of FML have already been shown in single applications. However, instead of using FML as a subsequent reinforcement of CFRP parts, they must be regarded as an individual material system to exploit their full potential. With regard to the variety of different fields of applications, the authors identified comprehensive research demand in the fields of the interface’s long-term durability, corrosion behavior, thermo-elastic properties, as well as in context of dimensional conformence.
The presented findings were essentially gained during the project no. 8 of the ’Schwerpunktprogramm SPP1712’ funded by the ’Deutsche Forschungsgemeinschaft’. The authors like to thank for funding and support. Another word of thanks is addressed to Salzgitter Mannesmann Forschung for providing material.
- Stefaniak, D.: Improving residual strength of unidirectionally reinforced plastic laminates by metal layering. Dissertation, Technische Universität Braunschweig, 2017Google Scholar
- Kolesnikov, B.; Herbeck, L.; Fink, F.: CFRP/titanium hybrid material for improving composite bolted joints. In: Composite Structures, 83 (2008), pp. 368–380Google Scholar
- Petersen, E.; Stefaniak, D.; Hühne, C.: Investigation of the transition zone for locally metal reinforced joining areas. ICCS18 - 18th International Conference on Composite Structures, Lisbon, Portugal, 2015Google Scholar
- Düring, D.; Weiß, L.; Stefaniak, D.; Jordan, N.; Hühne, C.: Low-velocity impact response of composite laminates with steel and elastomer protective layer. Composite Structures, 134 (2015), pp. 18–26Google Scholar
- Stefaniak, D.; Kappel, E.; Kolotylo, M.; Hühne, C.: Experimental identification of sources and mechanisms inducing residual stresses in multilayered fibre-metal-laminates. Euro Hybrid Materials and Structures, Stade, Germany, 2014Google Scholar
- Prussak, R.; Stefaniak, D.; Hühne, C.; Sinapius, M.: Residual Stresses in Intrinsic UD-CFRP-Steel-Laminates - Experimental Determination, Identification of Sources, Effects and Modification Approaches. Materials Science Forum. 825–826 (2015), pp. 369–376Google Scholar
- Stefaniak, D.; Kappel, E.; Kolesnikov, B.; Hühne, C.: Improving the mechanical performance of unidirectional CFRP by metal-hybridization. ECCM15 - 15th European Conference on Composite Materials, Venice, Italy, 2012Google Scholar
- Prussak, R.; Stefaniak, D.; Hühne, C.; Sinapius, M.: Experimental investigations on residual stress during the fabrication of intrinsic CFRP-steel laminates. Euro Hybrid Materials and Structures, Kaiserslautern, Germany, 2016Google Scholar